Ionizing ultra-strong acids with water molecules.

My previous posts have covered the ionization by a small number of discrete water molecules of the series of halogen acids, ranging from HI (the strongest, pKa -10) via HF (weaker, pKa 3.1) to the pseudo-halogen HCN (the weakest, pKa 9.2). Here I try out some even stronger acids to see what the least number of water molecule needed to ionize these might be.

Firstly what must surely be the ultimate acid H(CHB₁₁Cl₁₁), discovered by Christopher Reed[cite]10.1021/ja058581l[/cite] in 2006. This is so strong that it appears that it can even largely ionize itself; the form on the right (below) is the cationic acid, the form on the left is its anionic base. The proton itself is bridged[cite]10.5517/CCNBRWL[/cite] between the two in a manner similar to the structure of one form of HCl.4H₂O reported in the earlier post on the topic.

So it comes as no surprise^‡ to find[cite]10.14469/ch/191134[/cite] that just one water molecule can also ionize H(CHB₁₁Cl₁₁) to the anionic form (CHB₁₁Cl₁₁)^–.

How about triflic acid, CF₃SO₂OH, pKa -16), which is also a fair bit more acidic than HI? Here, only three waters are needed (ωB97XD/6-311++G(2d,2p) prediction) to ionise to triflate anion.[cite]10.14469/ch/191129[/cite]

So, if there a system which is ionised by precisely two water molecules I will record it here.

^‡Perhaps also no surprise is that one H₂S molecule can also perform this ionisation.[cite]10.14469/ch/191135[/cite] This leads us into another exploration, using molecules other than water to perform these ionisations.